1. Field of the Invention
The invention relates to a longitudinally adjustable gas spring for adjustable-height chairs, tables or the like, comprising a housing with a central longitudinal axis, defined by an exterior cylinder having an outside and an inside wall, a piston disposed in the housing for displacement in the direction of the central longitudinal axis, a piston rod disposed concentrically of the central longitudinal axis and secured to the piston and guided out of one end of the exterior cylinder, a valve for effecting a longitudinal adjustment of the gas spring, and a mounting section at the other end of the exterior cylinder.
2. Background Art
Longitudinally adjustable gas springs of the generic type are basically known from U.S. Pat. No. 3,656,593 and they are common practice in adjustable-height chairs. They are applied in such a way that the cylindrical section of the housing is disposed for axial displacement in a guide tube which is fixed to a pedestal of a chair. The outer free end of the piston rod is fixed to the bottom of the guide tube. The end of the housing, i.e. of the exterior cylinder, opposite to the exit of the piston rod is secured to a clamping device arranged on the bottom side of the seat of a chair or a table. Such a configuration of a longitudinally adjustable gas spring as a self-supporting element of a chair column or of a table column is likewise generally known and described for instance in U.S. Pat. No. 3,711,054. The mounting section, to be fixed to the securing device, of the external cylinder of the gas spring may be cylindrical as described in U.S. Pat. No. 3,711,054 or it may be in the form of a clamping cone, so that it simply has to be tightly inserted in a corresponding conical receptacle. This is common practice, too.
When the longitudinally adjustable gas springs of the generic type are used as self-supporting elements in particular in chair columns, the exterior cylinder is subject to reversed bending stresses that are highest where the mounting section passes into the purely cylindrical section, guided in the guide tube, of the exterior cylinder. These reversed bending stresses substantially result from the fact that as a rule, the users of chairs do not take a centered position relative the the central longitudinal axis of the gas spring when sitting on the chair, i.e. they apply considerable bending moments on the exterior cylinder of the gas spring, and that further to this, most users of chairs tend to move permanently while sitting on the chairs. So far, the risk of fracture due to the reversed bending stresses has been countered by the use of exterior cylinders of ever thicker walls and by employing steel grades of ever higher quality for the exterior cylinders. This was accompanied by correspondingly growing costs and still did not satisfy in terms of fatigue strength under reversed bending stresses.